Roll die assemblies for pilger mills

ABSTRACT

The utilization of smaller dies than those originally intended for a given size pilger mill is made possible for the first time by a new means of strengthening the necessarily small diameter shaft and by novel bearing blocks wherein smaller bearings are mounted off center. The smaller die assembly constructed according to this invention is completely and simply interchangeable with the original die sets and can be inserted in the pilger mill as a matter of course without modifications or adjustments to the machine proper.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus for cold forming metals, particularly to pilger mills for reducing the cross section of metal tubes by rolling between grooved roller dies and thereby making such tubes smaller and longer.

2. Background Information

A pilger mill contains two grooved rolls mounted on shafts and positioned one above the other. The dies are made to traverse back and forth in a reciprocating motion by a crank arm attached to the frame in which the dies and shafts are mounted. The stroke length of the motion is determined by the dimensions of the power driven crank and arm. Before this invention the built-in dimensions of any given size pilger would allow the use of only one size die diameter. U.S. Pat. Nos. 4,233,834 and 2,436,098, as well as others, describe the construction and operation of pilger mills.

The seamless tube industry uses pilger mills in a range of different sizes. Each size machine is designed for a single definite diameter size set of dies. For example, there are pilger mills that take fourteen inch dies, those that take eleven inch dies, those that take eight inch dies, and lately those that take five inch dies. Usually different mills are used for each step of the process as the tubes get smaller with each pass on the way to the final size.

In the process of making small tubes the starting material is relatively large extruded tube shell. The process is to reduce the size of the starting tube in steps, or passes, through successive machines. The final pass is usually made on a mill built for eight inch diameter dies because this was the smallest pilger made for many years past and numerous mills of this type are existing in tube production plants throughout the world. Five inch die diameter machines are now available but it would be unreasonable to replace existing eight inch die mills with the smaller machines due to the cost. Although it would be an advantage to the mills producing small tubes to be able to use the smaller dies.

The reason why smaller diameter dies are desirable for rolling small, thin wall tubes is obvious from observing the practices of the flat strip rolling industry. The thinner the strip, or sheet, to be rolled the smaller the diameter of the dies used. This is because at the point where the roll contacts and deforms the workpiece the unit forces are such that the steel roll deforms elastically and becomes flat at the metal strip deformation area. This deforming of the rolls requires extreme amounts of separating force and in some cases even prevents further thinning of the strip unless smaller diameter rolls are used. In rolling thin sheet or strip, rolls down to one inch in diameter are utilized. These principles also apply to rolling tubes in a pilger mill.

In the production of tubes, for example, a wall thickness of 0.030 inch is rolled in a pilger mill with five inch dies more readily than in a pilger mill with eight inch diameter rolls. For even thinner wall thicknesses the beneficial effect of smaller rolls is even more profound. If a tube production plant has invested in pilger mills requiring eight inch diameter dies, as many have, there is no choice but to use the eight inch dies. A huge investment in all new equipment is impractical.

In the course of producing tubes in a pilger mill the dies regularly wear out or fail by cracking, spalling, etc. and have to be replaced. The failed dies are replaced by removing a die assembly consisting of a shaft on which the die has been heat shrunk tightly on, roller bearings near each end of the shaft, bearing blocks in which the bearings are mounted, and a pinion gear at one end of the protruding shaft. Two such assemblies are required, one for the upper die and one for the lower die. These assemblies are usually prepared ahead of the need so as to be ready to change dies with a minimum of machine down time.

As described above, when small tubes are to be rolled smaller dies would be desirable. If a pilger mill has been designed and built for eight inch diameter dies, a simple substitution of smaller dies has heretofore been impossible for two reasons. The smaller shaft necessary to fit the inside diameter of the smaller die would not withstand the bending moments on the shaft brought about by the machine's lateral spread of bearing support members. Secondly the center-to center vertical distance of the smaller shafts would be so great in relation to the die sizes that the dies would not come together as they must for the operation of the process. As a result of these difficulties, those who have the larger pilger mills, such as the eight inch variety, have had to make thin wall tubes with dies that are too large in diameter for optimum mill operation.

3. Summary of the Invention.

This invention solves the problems of substituting smaller diameter dies into an existing pilger mill by changing the die configuration to strengthen the smaller shaft and by moving the centers of the two shafts closer together. Bearing blocks with eccentric bore are used to reposition the shafts closer together. These innovations can be incorporated into the usual die assemblies and inserted into the pilger mill with the same ease as an ordinary die change. No modifications or adjustments to the existing pilger mill proper is required in order to utilize this invention. It becomes unnecessary to acquire a costly new pilger mill when smaller dies are needed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the complete die assembly including the widened dies, the off center bearing blocks, the pinion gears, and the reduced size shafts.

FIG. 2 is a free body diagram illustrating the advantage of the transversely elongated die over the standard width die in adding resistance to bending of the shaft between a given distance between support bearings.

FIG. 3 is a cross sectional view of two of the bearing blocks positioned one over the other with the bearings inserted near the pass line center so as to accommodate the smaller dies.

FIG. 4 shows a comparison of the developed length path of an eight inch die and the smaller five inch die. Each die rolls a translated distance equal to the stroke of the pilger mill and in so doing rotates the necessary number of degrees to accomplish the travel.

DESCRIPTION OF THE INVENTION

The problem which this invention overcomes in retrofitting smaller diameter dies into pilger mills designed for larger size dies is mainly:

1. Any ordinary shaft of the usual type which will fit the smaller dies is too small in diameter to withstand the bending forces without excessive flexure.

2. The centers of the bearing blocks are vertically too far apart to allow the smaller dies to come together before the bearing blocks contact each other. These difficulties have heretofore prevented the use of but one size set of dies in any one pilger mill.

The transversely elongated, or wider, die, 1, in FIG. 2, is mounted on a shaft by heating to expand and slipping the die onto the shaft. Upon cooling the die shrinks and grips the shaft tightly. The die then becomes intrical with the shaft and reinforces it by increasing the effective cross section. This process is known as autofrettage.

The free body diagram in FIG. 2 shows that an ordinary die, 21 in the upper diagram, leaves more unreinforced shaft, 4, supported between the large pilger mill's widely spaced two bearing locations. This span of support cannot be altered without major modifications to the pilger mill proper. Therefore simply mounting a standard small die on its necessarily small shaft will result in an assembly which will not withstand the bending forces, represented by F in FIG. 2. The resisting forces are shown by arrows at the bottom of the bearings, 8. The diagrams in FIG. 2 are the lower die assemblies. The upper dies have forces in the opposite direction.

In order to utilize the smaller diameter dies they must be moved closer together than the original centerline of the larger shaft and dies would permit. It is necessary to keep the same outside dimensions of the bearing blocks so as to fit into the mill. The bearing blocks in this invention are the same size as previously used for the larger dies and are bored to receive the smaller bearings vertically off center as shown in 8 and 9, FIG. 3. In this embodiment it is necessary that, the outer diameter of the bearings, 8 and 9, FIG. 3, be slightly less than the diameter of the dies, 2 and 10, in FIGS. 1 and 3. This is so that when the bearing blocks are brought to their normal close position, as shown in FIGS. 1 and 3, the upper die, 1 in FIG. 1, and lower die, 1 a in FIG. 1, will come together. In FIG. 3 the arrows extending away from the bearings 8 and 9 show the direction of the forces caused by the rolling of tube. It is seen that even though the bearing block section is very thin in the opposite direction of the forces, there is sufficient section in the direction of the forces to structurally resist the parting pressure.

It should be understood that the new components constituting this invention, 1, 2, 3, 4, 10, 11, 8, 9, and 5 in FIG. 1 are such that they, as an assembly, can be taken in and out of a pilger mill as a matter of course in changing dies. This means that one pilger mill can be operated at one time with one size die set and at another time with a different size die diameter set. This has not been possible before.

In FIG. 1 the present invention is shown assembled as it appears placed in the pilger mill. The pinion gears, 5 and 6, at the ends of the shafts engage stationary rack gears permanently built into the original pilger mill. These pinion gears impart the turning motion to the dies when forced along the stationary racks. In order to impart nearly pure rolling of the die grooves, 22 in FIG. 1, on the tube being worked and not have sliding or dragging, the pinion gears must have slightly less diameter than the dies. This relationship results in the fact that the smaller dies with their smaller pinion gears fit exactly onto the existing racks with no adjustment. This is true because the pinion, 5 FIG. 1, moves up to its rack gear a distance equal to the amount that the smaller dies differ from the larger dies which also fit the machine. Pinion 6, FIG. 1 similarly moves down to its mating rack gear an equal amount.

A SPECIFIC EMBODIMENT

One of the most numerous pilger mill types in plants throughout the world uses eight inch diameter dies only. Most of these mills are used for the production of small thin wall tubes. In these cases a five inch diameter die would produce more satisfactory results as to tube dimensional control and in the rate of throughput.

The extent of the reciprocating travel, known as stroke length, is a built-in unchangeable feature of the pilger mill and is 15 inches long in the machines referred to above. When the eight inch dies roll and translates through the stroke length they rotate 216 degrees. This is illustrated in FIG. 4 as 19 for the eight inch die. This means only part of the die is used for working the tube.

Conversely, as shown in the lower part of FIG. 4, a five inch diameter die will rotate 350 degrees while traversing the same stroke length. The working groove, 22 in FIG. 1, in the five inch die can be extended to 350 degrees and still develop the same linear relationship to the workpiece. Therefore the same inside tool, known as the mandrel, can be used with either size die.

In FIG. 4, side views of eight inch (upper part) and 5 inch dies (lower), 11 is the outside diameter surface of the die, 12 is the groove, 13 is the inside bore of the die, and 14 is the unused portion of the eight inch die. In FIG. 4, lower, the corresponding parts of the five inch die are shown as 15, 16, 18, and 17, respectively.

The above is only one example of the application of this invention. For even larger pilger mills, which usually take large half dies of the 180 degree configuration, the same invention can be applied by use of smaller ring dies. The interchangeability of various size dies would also prevail in the larger half die machines when this invention is practiced.

With the ability to make use of more than one diameter set of dies in a single pilger mill, it is possible to make a multiple sequence of passes from a starting tube shell to smaller tube on the one machine. 

1. A die assembly for insertion into an already existing pilger mill said die assembly containing a set of smaller diameter dies than the mill was originally designed and built for, said dies being elongated transversely to increase their width to almost reach the inside members of the existing mill saddle and mounted by shrink fit on shafts, said shafts being supported by bearings mounted in bearing blocks with off-center bore, center line of said bearings being approximately as much closer to the center line of the pass as the difference said dies' radius and the radius of the larger dies originally designed for the mill.
 2. A die assembly as described in claim 1 having dies of any width less than the dimension between the saddle sides of the pilger mill.
 3. A die assembly as described in claim 1 in which a smaller ring die replaces a larger half die.
 4. A pilger mill die assembly with dies any diameter within 17 inches and 3 inches with the roll shafts strengthened by shrinking on the die and said shaft repositioned in the bearing blocks. 